Small TLP's (tension leg platforms) can be used to economically produce hydrocarbons from marginal offshore fields. Such TLP's can be made especially small compared to prior TLP's, by not providing the TLP platform with a derrick, but instead providing a servicing vessel with a derrick thereon. Whenever drilling, workover, or completion is required, which requires the use of a derrick, the vessel is moved to the platform. Whenever a storm approaches, the vessel is moved away.
U.S. Pat. No. 4,913,238 by Danazcko, describes a system of this type, wherein the vessel is towed to the TLP platform until the deck of the vessel lies above the platform and opposite sides of the vessel with pontoons thereon, lie on opposite sides of the platform. The vessel is separately moored, by catenary chains, and performs operations requiring its derrick. While such a system enables the use of relatively small TLP's, it has several disadvantages. One disadvantage is that the vessel can be used only in calm seas, or else it may vigorously strike the platform and damage both of them. Also, calm seas are usually required to operate the derrick on a platform or on the separate vessel, to assure that the platform will remain almost directly over the wells to be drilled or serviced. An offshore production system, wherein a vessel that carried a derrick to and away from a small platform could be more quickly coupled to the platform and could be used in moderately adverse weather, would be of considerable value.